Fan controls for variable air flow systems with multiple terminals waste energy if the variable air flow systems do not turn down the fan speed to the lowest speed that satisfies the various terminals. Fan control systems are designed to find that lowest speed. Systems that are not optimized run faster, consuming excess energy, or run too slow and fail to satisfy the terminals.
To minimize fan power, some systems collect data from the terminal controllers. The data indicates the state of the flow controller (e.g., the position command to the flow control damper). The fan controller adjusts fan speed or duct pressure to get at least one damper close to open. Other systems evaluate sufficiency of the fan output in each terminal controller, determining if the flow controller is satisfied or not, and indicating the level of satisfaction to the fan controller. The satisfaction is usually a binary signal or discrete signal expressing several levels of satisfaction. Other systems use flow setpoint values from the terminals and a duct model to calculate the pressure necessary to satisfy the terminals. None of these approaches applies to a system that uses a venturi air valve as a flow control device. The first two approaches rely on knowing the position of the adjustable obstruction to air flow. In a venturi air valve, the cone moves on a shaft, independent of the control system. The position may not directly relate to needed fan speed or pressure. For the model approach, the existing models do not represent the venturi valve.
For systems with a venturi valve, a pressure sensor (or switch) measures the pressure across the venturi valve to determine if the pressure is sufficient. This measurement is used to control fan speed, but requires a significant investment in additional pressure transmitters (e.g., switches), wiring, and programming for each venturi valve, along with the added burden to periodically recalibrate each of those pressure indicating devices.